To meet the demand for higher integration density and operating speed of LSIs, the effort to reduce the pattern rule is in rapid progress. The wide-spreading flash memory market and the demand for increased storage capacities drive forward the miniaturization technology. As the advanced miniaturization technology, manufacturing of microelectronic devices at the 65-nm node by the ArF lithography has been implemented in a mass scale. Manufacturing of 45-nm node devices by the next generation ArF immersion lithography is approaching to the verge of high-volume application. The candidates for the next generation 32-nm node include ultra-high NA lens immersion lithography using a liquid having a higher refractive index than water in combination with a high refractive index lens and a high refractive index resist film, extreme ultraviolet (EUV) lithography of wavelength 13.5 nm, and double patterning version of the ArF lithography, on which active research efforts have been made.
As the feature size reduces, image blurs due to acid diffusion become a problem. To insure resolution for fine patterns with a size of 45 nm or less, not only an improvement in dissolution contrast is important as previously reported, but control of acid diffusion is also important as reported in Non-Patent Document 1. Since chemically amplified resist compositions are designed such that sensitivity and contrast are enhanced by acid diffusion, an attempt to minimize acid diffusion by reducing the temperature and/or time of post-exposure bake (PEB) fails, resulting in drastic reductions of sensitivity and contrast.
A triangular tradeoff relationship among sensitivity, resolution, and edge roughness has been pointed out. Specifically, acid diffusion must be suppressed to achieve a resolution improvement whereas a short acid diffusion distance leads to a loss of sensitivity.
The addition of an acid generator capable of generating a bulky acid is an effective means for suppressing acid diffusion. It was then proposed to incorporate recurring units derived from an onium salt having a polymerizable unsaturated bond in a polymer as acid generator. Patent Document 1 discloses a sulfonium salt having a polymerizable unsaturated bond capable of generating a specific sulfonic acid and a similar iodonium salt. Patent Document 2 discloses a sulfonium salt having sulfonic acid directly attached to the main chain.
A tradeoff relationship between sensitivity and edge roughness has been pointed out. For example, Non-Patent Document 2 describes that sensitivity is in inverse proportion to edge roughness. It is expected that the edge roughness of a resist film is reduced by increasing the exposure dose to reduce shot noise. Non-Patent Document 3 describes a tradeoff between sensitivity and roughness in the EUV lithography in that a resist material containing a more amount of quencher is effective in reducing roughness, but suffers from a decline of sensitivity at the same time. There is a need to enhance the quantum efficiency of acid generation in order to overcome the problem.
With respect to the acid generating mechanism triggered by EB exposure, Non-Patent Document 4 reports that PAG releases acid through the mechanism that a polymer is excited by exposure so that electrons migrate to the PAG. Since the irradiation energy of EB or EUV is higher than the threshold value (10 eV) of ionization potential energy of a base polymer, it is presumed that the base polymer is readily ionized. An exemplary material of accelerating electron migration is hydroxystyrene.
It is reported in Non-Patent Document 5 that poly-4-hydroxystyrene has a higher acid generation efficiency in EB exposure than poly-4-methoxystyrene, indicating that poly-4-hydroxystyrene provides for efficient migration of electrons to PAG upon EB exposure.
Non-Patent Document 6 proposes a material obtained through copolymerization of hydroxystyrene for increasing the acid generation efficiency by electron migration, a methacrylate of PAG having sulfonic acid directly bonded to a polymer backbone for suppressing acid diffusion, and a methacrylate having an acid labile group.
Since hydroxystyrene has a phenolic hydroxyl group which is weakly acidic, it is effective for reducing swell in alkaline developer, but causes to increase acid diffusion. On the other hand, a methacrylate having lactone ring as the adhesive group is widely employed in the ArF resist composition. Since this methacrylate has high hydrophilicity and no alkaline solubility, it is ineffective for reducing swell, but effective for suppressing acid diffusion. A combination of hydroxystyrene and lactone ring-bearing methacrylate as the adhesive group can establish a fairly good balance among sensitivity improvement, swell reduction, and acid diffusion control, but is still insufficient.
Copolymerization of hydroxyphenyl methacrylate with lactone ring-bearing methacrylate and optionally methacrylate of PAG having sulfonic acid directly bonded to the polymer backbone is effective for forming resist compositions having a high sensitivity, high resolution, and controlled acid diffusion. An attempt to increase the content of hydroxyphenyl methacrylate is effective for further increasing the sensitivity. However, as the content of hydroxyphenyl methacrylate increases, alkaline solubility increases, indicating that the pattern will undergo a film thickness loss and eventually collapse.
Since iodine is highly absorptive at wavelength 13.5 nm, it generates secondary electrons upon exposure to which the acid generator is sensitive, indicating a higher sensitivity. For example, iodonium salts have a higher sensitivity than sulfonium salts. Patent Document 3 shows an iodized polymer, specifically a polymer comprising recurring units having an iodized aromatic group. Since the aromatic group is electron absorptive, 25 secondary electrons generated by iodine are taken into the aromatic group, leading to a low sensitizing effect. To develop a resist composition having higher sensitivity and resolution, a substance capable of efficiently exerting a sensitizing effect is desired.